Once the fuels are in your body, you have to reprocess them in order to utilize them to provide the energy and building blocks for daily survival and movement.
The energy cost of all of these physical and chemical changes in your body is called your metabolic rate. Metabolic rate is usually measured in energy units, calories, or kilojoules, which are the same units used to measure the amount of potential energy in the food you eat.
Basal Metabolic Rate
The basal metabolic rate represents the energy cost of body maintenance. If you lazed around in bed all day you would still require energy for the physiological processes that work non-stop around the clock.
For example, it takes energy to transform food to energy; to make new compounds such as hormones, enzymes, and proteins; to grow new tissues such as muscles and bones, or to repair existing tissues; to keep your heart beating to power the body’s transportation system; or to keep your lungs pumping to get oxygen into your body.
Lifestyle Metabolic Rate
Once you do get out of bed, you’re going to be using up even more energy during your normal daily activities. The lifestyle metabolic rate represents the extra energy needed to get you through the day without fatigue or ill-health. The person who sits at a desk all day is going to use up less energy than the person who is a laborer on a building site.
If both these people begin with the same level of body fat and eat the same amount of food, that is they have the same energy intake, then the desk-bound person will get fatter while the laborer stays lean.
The simple little relationship is called the “energy equation”. On one side of the energy equation is your energy intake, and on the other side is your energy output.
If you take in more energy than you use up, then you’re in positive energy balance and your body will store the excess energy as fat in adipose tissue.
If you take in the same amount of energy that you are using up, then you’re in a neutral energy balance, and your level of body fat will stay the same.
If you take in less energy than you use up, then you’re in a negative energy balance, and you’ll have to draw on your fat stores to make up the shortfall, so you’ll get leaner.
Manipulating your energy balance is that simple, but the ways of ensuring that your body meets a negative energy balance by using your body fat stores rather than your blood and muscle sugar stores are a little bit more complicated.
So how does our desk-bound person avoid getting obese and putting themselves – at risk of the associated health problems? The answer is simple: they exercise!
Exercise Metabolic Rate
Together with your basal metabolic rate and your lifestyle metabolic rate goes the third contributor to daily energy consumption in your body — exercise metabolic rate.
Exercise metabolic rate is the amount of energy you expend in recreational activities, such as watching TV, or going for a walk, or digging in the garden, or playing or training for sport, or going to exercise classes, or climbing a mountain, or going surfing, or doing woodwork in your workshop.
The study of exercise energy costs has undergone a shift in emphasis in recent years with the revival in interest in exercise physiology. Physiology, the study of the metabolic processes of the body, has been dominated for the past 40 years by sports physiologists concerned with ways of producing the maximum energy output for maximum performance.
However, exercise physiology is concerned with the workings of the average person, not the elite athlete, and the average person exercises for different goals. An example of the results of this shift in emphasis to exercise physiology is the rethink of relative exercise intensities. Until recently people exercised as hard as they could drive their bodies in “aerobic” activities such as running, exercising to music, or circuit training.
Sports physiologists thought this was good as it improved ultimate potential, but exercise physiologists thought it wasn’t so good as the main fuel used was body stores of glycogen rather than body fat.
What was happening was that people were getting fitter, but not getting leaner. To get leaner, you have to work at lower intensities for longer periods. This gives the same energy cost but utilizes more fat as a preferred fuel.
This simple little recognition of a basic physiological concept has revolutionized the fitness industry. New classes, such as “New Body”, target body fat metabolism. There are rows and rows of “aerobic” machines such as steppers, rowers, bikes, and treadmills instead of rows and rows of weight machines. There are fewer injuries, as intensity has gone down and injury rates are proportional to intensity.